The invention is particularly applicable to electron microscopes of the type in which a specimen to be analysed is held within a gaseous environment contained in a specimen chamber and to the analysis of specimens which may outgas or shed particles in response to the impact of the primary electron beam. The gaseous environment avoids or mitigates certain of the problems which arise from analysing a specimen in high vacuum. For example, the environment may prevent or inhibit degradation of biological specimens, and can help to dissipate surface charges which would otherwise accumulate on a non-conducting specimen, to the detriment of image resolution.
Some scanning electron microscopes can obtain images of a specimen in a gaseous environment in which relatively high pressures are maintained. Such pressures can be at least the vapour pressure of water at room temperature, and in some cases the facility to operate at pressures up to one atmosphere in the specimen chamber is claimed.
However, a relatively high vacuum needs to be maintained in the electron optical column of the microscopes, and to that end the microscopes are generally equipped with differential pumping stages comprising at least two spaced apart pressure limiting apertures (through which the beam passes) which operate in conjunction with one or more vacuum pumps connected to the space between the apertures and the region above the upper pressure limiting aperture. Most of the gas which passes from the specimen chamber through the lower aperture is pumped out of the space between the two apertures. Some gas may escape from this region through the upper limiting pressure aperture, but this will then be removed by the pump connected to the region above that aperture.
An electron optical column may include further pumping stages in order to achieve a higher vacuum in the column, depending on the type of electron source used in the column.
The two pressure limiting apertures will normally be mounted on or in the final electromagnetic lens assembly of the electron optical column, which will have scanning coils or electrodes located just upstream (in the beam direction) of the upper pressure limiting aperture. This means that the lower aperture may limit the field of view of the electron microscope. In addition, the relatively high number density of gas molecules in the specimen chamber limits the mean free path of electrons in the beam, and hence correspondingly limits the working distance of the microscope.
Many microscopes can operate in a “variable pressure” mode (VP mode), in which lower pressures in the specimen chamber are used. In such a mode, the microscope can have a greater working distance and the lower of the two pressure limiting apertures may be omitted or (in the case of a reconfigurable microscope) removed. The field of view is no longer limited by the lower aperture, but molecules or particles ejected from the specimen by the beam can accumulate around the remaining pressure limiting aperture to cause contamination of the latter or of the column parts located upstream (relative to the beam direction) of the aperture. Such contamination of the aperture may, for example, give rise to the accumulation of electrostatic charges that are detrimental to image quality.
Similar accumulations can occur on the lower aperture of a microscope operating in an ESEM mode, but that aperture can readily be removed for cleaning. However, the aperture of a microscope operating in variable pressure mode may be recessed within the final lens assembly, and so may be relatively inaccessible.
U.S. Pat. No. 3,156,811 and GB 1120864 show particle beam aparatusses which are designed for performing various operations (for example drilling or welding) on workpieces, and which use a flow of gas through the final orifice through which the particle beam passes to attempt to keep that orifice clean. In these cases, however, the orifice has to be incorporated into an attachment for the final lens which attachment is specifically designed to provide the gas flow and which increases the minimum permissible distance between the actual lens and the workpiece. It is also believed that, despite the flow of cleaning gas, contaminants can accumulate on the lower surface of an attachment, in the region of the orifice.